The map data update systems disclosed in JP-A No. 2002-279437 and JP-A No. 2005-122003 have been known as an apparatus to update map data in units of several areas. The apparatus of JP-A No. 2002-279437 is characterized by that, when updating map data in units of sever al areas, the connection between roads is maintained by sending simultaneously only the connection information to resolve any disconnection across areas. The apparatus of JP-A No. 2005-122003 is characterized by that it is determined in advance whether or not a disconnection of roads is detected at the border between areas and this information is obtained before the start of updating.